Considerable difficulties in the evaluation of velocity information occur in areas of complex subsurface structure due to the presence of diffractions, sidewipe reflections, and multiples. Conventional seismic processing up to final time migration cannot adequately overcome these difficulties but is accepted as a key element for velocity macro model estimation based on the identification of major reflectors. This is one reason while there is still ongoing research on the derivation of migration velocities.
Conventional velocity analysis amounts to preprocessing the data and scanning for hyperbolic moveout. For horizontally layered media, the preprocessing consists of simple sorting into common midpoint (cmp) gathers, hereinafter called RAWCMP, while in general, more sophisticated preprocessing is needed to generate data sets which mimic cmp sorted field data. The RAWCMP gather contains velocity information encoded in the traveltime curves of reflection events related to the normal rays but biased and out of focus for dipping reflectors (FIG. 1A)
Forel, D. and Gardner, G. H. F., describe in "A three-dimensional perspective on two-dimensional dip moveout", 1988, Geophysics 53, 604-610, an algorithm which is able to convert any recorded seismic traces to an equivalent set of zero-offset traces. This dip moveout (DMO) algorithm can be applied to multifold inline data no matter what the variations in azimuth and offset may be. Due to the dip moveout correction to multiple offset, the velocity bias can be removed while a created DMOCMP (Dip Moveout Common Midpoint) gather now focusses on the common reflection points (FIG. 1b). DMO is viewed here as an operation which brings the data from an arbitrary initial configuration to preselected cmp. locations on the line segment from the source to the receivers without requiring any use of velocity. There is a trace in the target crop gather at a location where a velocity analysis is needed, after a linear squeezing of its time axis and a redefinition of its offset coordinate, both depending on the original offset and the distance of the traces cmp location to the target cmp.
A similar offset redefinition technique is disclosed by Hubral, P. and Krey, T. 1980, "Interval velocities from seismic reflection time measurements", SEG, to create migration-before-stack gathers. All traces having midpoints within a migration of prespecified radius centered at the target crop constitute these gathers.
A drawback of the DMO-technique is that the information in a DMO common midpoint stems from subsurface points located along normal rays which are normal to the corresponding layer. Thus, in case of dipping layers, the vertical projections of the subsurface points relating to the seismic events gathered in one common midpoint do not coincide. So, in order to bring the recorded seismic events into their right positions a migration has to be performed. Up to now for successful migration a detailed knowlegde of the velocities is required.